Don't rely on rand details for testing t/libravatar.t

[schwern]

Libravatar::URL uses rand() to pick a hostname out of a federated list of servers. t/libravatar.t sets srand to be able to predict how Libravatar::URL will behave. Unfortunately sometimes Perl uses different implementations of rand that produce different results.

Make the test deterministic some other way.

• Test::MockRandom is probably the simplest.
• See https://rt.cpan.org/Ticket/Display.html?id=71064#txn-980048 for another solution.

See http://matrix.cpantesters.org/?dist=Gravatar-URL%201.06;os=mswin32;perl=5.18.2;reports=1 for examples of failures.